Flowmeter Arcing Phenomenon

[tc7]

I have searched and found many good posts on static discharge, but nothing which relates to my situation:

On our hydraulic test benches we use a glass tube, float type flowmeter; fluid is Mil type hydraulic and non-flammable. We occasionally get a report of arcing from this flowmeter, but reports and descriptions of the event are non-specific except that "an arc (or spark )was seen inside the tube between the float and the float stop". I am told in one instance that the glass tube shattered.

I was tempted to jump to a conclusion that we simply had a bad metal-to-metal contact between the flowmeter housing and the back panel, but all connections look clean. I have other reports which claim a ground wire was added between the flowmeter metal case and an earth ground with no apparent effect on preventing the arc. The arc cannot be duplicated upon request, and so I am at a loss to view the phenomenon myself.

1. what is the possible cause of this charge buildup?
2. how does fluid velocity effect size of the charge buildup? (Typical flow measurements are ~20gpm)
3. is it possible to strap a cable to the glass (externally)and connect it to the chassis ground, will this do any good?
4. what else can I do to prevent the arc?

Any advice or explanations of this phenomenon are welcome.
Please help and thankyou.
Tom

 

[jmw]

Static is a concern for medical gas (e.g. O2)use and such VA meters are often specially treated. Perhaps they can apply a similar solution for your application.
I would suggest that you raise the question with your meter supplier.

JMW

http://www.ViscoAnalyser.com

 

[hacksaw]

you need to use a metal tube rotameter or a transmitting device (vortex shedder, orifice,coriolis, etc.) for that service

charge build up in insulating fluids is a well known problem especially at high fluid velocities.

 

[quark]

Have you thought of grounding metal rings which are generally used with lined flowmeters?

 

[tc7]

quark-
no I have not thought of grounding metal rings, but where in the flow meter assembly would these be located? I assume the charge is building up on the float ( which is anodized aluminum) and then arcing to the metal flow meter base. Where whould such a ring be incorporated?

 

[hacksaw]

grounding rings are used in non-metallic piping. there are not useful in metal piping as long as the joints are properly bonded.

what fluid velocity (pipe) and what d/p across the meter?

 

[tc7]

I was hoping this problem would fix itself while I was away, but no such luck.

Hacksaw-
The velocity is quite low I believe, at 20-24 GPM I estimate the velocity is only ~5 ft/sec or so. We can't guess at the dP across the meter, but we think it is nil compared to the line pressure of ~100-120 psi.

Quark-
I have looked at employing grounding rings. Do you have any data on the ground impedence these rings should be connected to?


Thanks for all your thoughts.
Tom

 

[hacksaw]

Grounding rings are only used (and useful) in non-conductive piping.

20 gpm is 4.2 fps in 1-1/4" piping. Are you really using a glass tube meter that large and at a pressure of 120 psi?

Most operating plants won't permit such a meter for safety reasons. The arcing explanation is not supported by experience. The more common experience is improperly designed installations and hydraulic shock.

 

[quark]

My initial comment seems to be superficial after Hacksaw's continuous insistence about the root cause. However, Grounding rings are installed even with metal piping when lined meters(particularly mags)are used. You can strike out this idea if your fluid in non conductive.

I am not knowledgeable about the electrical question. You can get good answers from one of the electrical forums.

Regards,

 

[pennpoint]

I can speak from my own experience: In my past 15 years, I designed a fluid cart, utilizing FC-77 fluid, a 3M product that exhibits high dielectric properties. During testing we witnessed significant arcing in the supply flow meter (rotometer). The solution was to attach a short length of SST ball chain to the riser (float) and to the collar of the tube. (Attachment by braising). The result was no more arcing, and no significant effect upon flow indication.

I hope this helps your situation,

Best Regards

pennpoint

 

[hacksaw]

Metal tube rotameters work fine and provide the continuity you need.

What is puzzling is the oft made assumption that rotameters are a cost effective solution in the 1-2" size range. Hydraulic shock is the biggest worry. Attaching weight to the float is a no-no as it changes the meter calibration, but if it works who complains...

The arcing in hydraulic fluids is well known and observed personally, but what is surprizing is the report of it at low fluid velocities.

To Quark: it is okay to bond the adjacent piping for electrical continuity. It is just that the use of grounding rings is not necessary. That said, you will see them in use, typically as part of an an installation standard meant to cover all cases.

 

[kellnerp]

Arcing inside a hydraulic system may be possible. I ran across a paper on putting out fires in petroleum storage facilities. It turns out that as the water leaches out of the foam used to blanket the fire in those big tanks the water droplets carry a charge as they drop through the oil. Under certain conditions there can be enough static buildup to initiate an arc and restart the fire.

This suggests that there might be moisture in your system which can carry a charge. Just a far out thought.